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Temperatures are climbing. With the dog days of summer just around the corner, the perfect time to shop for a new air conditioner is now.

There are numerous factors to consider when you’re shopping for an air conditioner, including whether your home can accommodate a ductless mini-split system, a built-in wall assembly, a window unit, or a portable model. But all air conditioners have one thing in common: They must be sized properly in order to effectively lower the temperature and remove excess moisture from the air, resulting in a comfortable, cool indoor environment.

When you’re shopping for an appropriately sized air conditioner, a number of factors come into play, including the general climate and average summer temperatures in your region of the country; the square footage of the room or rooms to be cooled; the installation location in the wall, window, or ceiling; the number of people typically occupying a room; and the amount of insulation in the home. An air conditioner that is too small for a given area will not be able to cool the space efficiently, while an air conditioner that is too large will tend to cycle on and off too rapidly, wasting energy and impairing the unit’s ability to remove humidity from the room.

Air conditioners are rated by their seasonal energy efficiency ratio (SEER), and their capacity is expressed in British thermal units (BTUs). The BTU rating gives an indication of how quickly and effectively a particular unit can cool the room where it is located. Most home air conditioners sold in the United States range from around 5,000 BTUs to more than 20,000 BTUs.

LG mini-split air-conditioning unit from SupplyHouse.com

“The BTU—or British thermal unit—is the amount of heat required to raise or lower 1 pound of water 1 degree Fahrenheit,” points out Daniel O’Brian, a technical expert from online retailer SupplyHouse.com. “Since we measure all of our heating and air-conditioning units in BTUs, it’s important to know how many BTUs your living space requires so you don’t buy a unit that is too large or too small. Another common measurement that heating and air-conditioning units are assigned is ‘tonnage.’ But don’t let this confuse you—one ton is just 12,000 BTUs!”

Correctly sizing an air conditioner for a given room requires a tape measure and a few simple calculations. The first step is to determine the size of the room where the unit will be installed. SupplyHouse.com offers a handy sizing calculator to determine the correct amount of BTUs needed for a given room or set of rooms. The calculator requires the dimensions of a room, in length and width; the type of room, such as kitchen or bedroom; the number of people typically in the room; and finally, the exposure of the room—whether it is very sunny or shaded. Once you plug in the appropriate information, the calculator will determine the correct size air conditioner in BTUs. If you are cooling two adjacent areas, or if your room is odd-shaped, determine the square footage of each space as if it were a separate room, and then add the two measurements together to get a total amount of BTUs.

Photo: Energy.gov

According to the U.S. Department of Energy, another rating that homeowners should consider when purchasing a new room air conditioner is the Energy Efficiency Rating, or EER, which represents the cooling capacity of a unit in BTUs per hour divided by the watts of power consumed at a specific outdoor temperature (usually 95 degrees Fahrenheit). The EER rating is found on the yellow Energy Guide label on the air conditioner, and it typically ranges from 8 to 11.5. An EER rating of 10 or higher is the most efficient and will yield the highest savings on monthly electric bills. A higher EER also helps the environment by reducing greenhouse emissions.

Online retailer SupplyHouse.com offers a large selection of mini-split air conditioners and accessories from the top manufacturers in the industry, and features a variety of informative tools and instructional videos on its Web site. Visit them here.

This post is sponsored by SupplyHouse.com. Its facts and opinions are those of BobVila.com.